There are a number of traditional ways of forming images on a printing plate. These include, but are not limited to, molding, photoflash imaging, and thermal imaging. The present invention is directed broadly to the area of thermal imaging or thermography. Thermal imaging or thermography is a recording process wherein an image is generated by the use of image wise modulated thermal energy. Traditional, "direct thermography" refers to a method whereby a visible image pattern is formed by the image wise heating of a recording material containing matter that by chemical or physical process changes color or optical density. Most of the direct thermographic recording materials are of a chemical type. On heating the recording material to a certain conversion temperature, an irreversible chemical reaction takes place and a colored image is produced.
In recent years, the field of manufacturing or producing printing plates such as rubber stamps has experienced rapid advancement, especially in the methods used to fabricate the stamps. Resinous relief printing plates, planographic printing plates, and intaglio printing plates all formed using photosensitive resins now enjoy widespread use. In addition, a number of methods are known for forming printing plates using stencil images. These methods generally involve the use of a thermal printer or a wire dot printer to form a stencil image on a sheet, which is then utilized as a printing plate.
Another method of forming relief plates involves forming a printing plate by heat pressing a photosensitive resinous printing plate with an image plate, thereby forming a relief image on the photosensitive material. This method involves a complicated process for producing either a metal printing plate or a photosensitive resinous printing plate. In addition, the use of a press machine or heated roller is required for this process. Further, positioning the embossing plate properly on a plate prior to pressing can be challenging to even those highly skilled in the art, and deviation from the proper position can readily occur.
As the use of relatively simple printing systems has expanded, the need for a simple, yet reliable, method of printing a printing plate has come into existence. In an attempt to address this need, U.S. Pat. No. 5,665,524 (which is hereby incorporated herein in its entirety by reference) describes a printing plate which is formed from an open celled thermoplastic medium wherein the open cells are sealed upon exposing to energy rays. The methods described in this reference generally require the presence of a negative to block photorays which results in a portion of the photosensitive thermoplastic medium remaining unhardened by blocking the energy rays, and a separate portion which is hardened by exposure to the energy rays. The exposed portion forms a background of the image to be formed on the stamp. The background portion prevents the transfer of ink from the thermoplastic foam to the receiving medium (i.e. paper) in these regions. It is speculated in U.S. Pat. No. 5,665,524 that an expensive laser system may be driven to seal predetermined portions of the foam to form the background image. In addition, this reference places a great deal of emphasis on forming the open-cell image in the same plane as the background image by sealing only a surface layer of open cells, thereby forming a plate having little or no relief.